diff --git a/src/index.js b/src/index.js
index b94037e..d8aee58 100644
--- a/src/index.js
+++ b/src/index.js
@@ -6,33 +6,37 @@ import { insertStyle } from './style.js';
 import mkdirp from 'mkdirp';
 
 /**
- * Write to a file even if its directory does not exist
+ * Appends to a file even if its directory does not exist
  * @param {String} path the path of the file write to
  * @param {String} contents contents of file
- * @param {Function} cb callback function when write action finish
  */
-const writeFile = (path, contents, cb) => {
-    mkdirp(dirname(path), function (err) {
-        if (err && typeof cb === 'function') return cb(err);
+const appendToFile = (path, contents) => {
+    return new Promise((resolve, reject) => {
+        mkdirp(dirname(path), function (err) {
+            if (err) {
+                reject(err)
+            }
 
-        fs.writeFile(path, contents, cb);
+            fs.appendFileSync(path, contents)
+            resolve(r);
+        });
     });
 }
 
 let renderSync = (code, option) => {
     return less.render(code, option)
-        .then(function(output){
+        .then(function (output) {
             return output.css;
-        }, function(error){
+        }, function (error) {
             throw error;
         })
 };
 
 let fileCount = 0;
 
-export default function plugin (options = {}) {
+export default function plugin(options = {}) {
     options.insert = options.insert || false;
-    const filter = createFilter(options.include || [ '**/*.less', '**/*.css' ], options.exclude || 'node_modules/**');
+    const filter = createFilter(options.include || ['**/*.less', '**/*.css'], options.exclude || 'node_modules/**');
 
     const injectFnName = '__$styleInject'
     return {
@@ -51,33 +55,35 @@ export default function plugin (options = {}) {
                 options.option['filename'] = id;
                 options.output = options.output === undefined || options.output === true ? 'rollup.build.css' : options.output;
                 if (options.plugins) {
-                  options.option['plugins'] = options.plugins
+                    options.option['plugins'] = options.plugins
                 }
 
                 let css = await renderSync(code, options.option);
 
-                if(options.output&&isFunc(options.output)){
+                if (options.output && isFunc(options.output)) {
                     css = await options.output(css, id);
                 }
 
-                if (options.output&&isString(options.output)) {
-                    if(fileCount == 1){
+                if (options.output && isString(options.output)) {
+                    if (fileCount == 1) {
                         //clean the output file
                         fs.removeSync(options.output);
                     }
-                    writeFile(options.output, css);
+                    await appendToFile(options.output, css);
                 }
 
                 let exportCode = '';
 
-                if(options.insert!=false){
+                if (options.insert != false) {
                     exportCode = `export default ${injectFnName}(${JSON.stringify(css.toString())});`;
-                }else{
+                } else {
                     exportCode = `export default ${JSON.stringify(css.toString())};`;
                 }
                 return {
                     code: exportCode,
-                    map: { mappings: '' }
+                    map: {
+                        mappings: ''
+                    }
                 };
             } catch (error) {
                 throw error;
@@ -86,18 +92,18 @@ export default function plugin (options = {}) {
     };
 };
 
-function isString (str) {
-    if(typeof str == 'string'){
+function isString(str) {
+    if (typeof str == 'string') {
         return true;
-    }else{
+    } else {
         return false;
     }
 }
 
-function isFunc (fn){
-    if ( typeof fn == 'function' ){
+function isFunc(fn) {
+    if (typeof fn == 'function') {
         return true;
-    }else{
+    } else {
         return false;
     }
-}
+}
\ No newline at end of file